new ActiveFedora::Auditable mixin - provides access to Fedora audit trail

unit tests for ActiveFedora::Auditable

Added FOXML fixture for AF::Auditable unit tests

Added Auditable to eager autoload modules

made fixture load work

Requiring Rubydora 1.6; ActiveFedora::Auditable updated
Merge pull request #58 from projecthydra/audit-trail

new ActiveFedora::Auditable mixin - provides access to Fedora audit trail